Halifax and Lloyds TSB customers have had their cards rejected and been left unable to use some ATMs across the country today.
Some customers reported having to leave their shopping behind while others said they have been left unable to access their bank accounts, just before the weekend starts.
A Halifax spokeswoman said the hitches were the result of a "systems failure", which has affected the group's telephone systems as well as ATMs and online services.
She said: "The systems are now up and running and we are working through the backlog. The entire backlog will be processed today."